This invention relates to breathing apparatus and is especially but not exclusively concerned with diving headgear for use in return-line or push-pull diving systems in which pressurized breathable gas is fed to the headgear through a supply hose, used gas is withdrawn from the headgear through an exhaust hose and pressurized, and the pressurized gas is recycled to the headgear through the supply hose. Such headgear usually comprises a helmet, an oral nasal mask in the helmet, a continuous free-flow supply valve on the helmet, and an exhaust regulating valve on the helmet actuable by the breathing of the diver to permit the withdrawal of the used gas by suction through an outlet opening in the helmet.
As the breathable gas is usually a helium/oxygen mixture, return-line diving systems have the considerable economic advantage of allowing re-use of expensive helium. However, existing return-line or push-pull diving systems have serious disadvantages. Thus, with reduced pressure in the exhaust hose to ensure efficient removal of the used gas from the helmet, failure of the exhaust regulating valve due say to jamming arising from close tolerances or to failure of sliding seals will cause lung "squeeze" which can prove fatal. Moreover, the valve systems on the helmets have hitherto been unable to meet the criteria of (a) adequate safety back-up combined with high gas-flow rates for good lung ventilation and (b) high mechanical advantage with consequent low sensitivity to across-the-valve pressure fluctuations, since the provision of large openings required for high flow rates normally results in a reduction in the mechanical advantage of the valve system.
The object of the present invention is to provide diving headgear with a valve system by virtue of which the aforesaid disadvantages in the existing return-line or push-pull diving systems are obviated or mitigated.